I don't know where you are based but the Pentax customer service I received (in the UK)when my K10D developed a fault (possibly due to the alcohol induced meeting with the floor) was first rate.
The camera was sent to Pentax, repaired and returned within 7 days - free of charge.
I believe that in New Zealand Pentax are a bit thin on the ground
In terms of what you can achieve with a Pentax and some old 'inferior' lenses feel free to take a peek at my flickr site http://www.flickr.com/photos/10126769@N05/
- maybe not the best example but an example none the less.
There are many excellent older lenses around at reasonable prices if you are patient.
I don't know the Nikon D3100 but in recent years the entry level Nikons have had no af motor in the camera and so all lenses have to contain the motor (if you want af) and they have the price to go with them - worth knowing this before purchasing as for example, as far as I am aware a budget telephoto such as the sigma or Tamron 70-300 suddenly become twice or three times the price when you need the one with the motor in it
- at this point the Pentax or other bodies containing an af motor may start to look more favourable
My advice would be to go and handle each camera - you may find that you just love one more than the other - you may not.
Also ask what you want/ expect to achieve with your camera - my K10D works well with a varierty of lenses and even sets the exposure automatically whilst using m42 lenses etc, others you have to press the green button for this - I believe that othe rmanufacturers don't have the same compatibility but I could be wrong.
Sony carries over a Konica/ minolta moutn and also has a number of older lenses to play with.
My personal thought on the matter is that I can buy cheap old lens of a given length to play with before deciding if I want to upgrade to a much more expensive modern option - it's nice to have the choice.
Nikon of course have a more extensive lens range than Pentax, most notably at the longer end, however, I don't have £6,000 for a Nikon 500mm f4 or £3,849 for the (Pentax compatible) Sigma 500mm F4.5 so this does not matter to me - if it does to you and you may have that kind of cash for that kind of lens you know which camp to park up in
Hope this helps
PS don't be swayed by the badge, pick whichever camera suits you best and of course check out all of the options
check out as many as you can - you never know a Canon maybe the one that feels 'best in hand' for you - who knows